With the continued miniaturisation of electronics components and structures, many of which utilize a screen printing process, there is a need to be able to print finer pitch deposits which have smaller area ratios, and with an acceptable transfer efficiency and area coverage.
The area ratio is the ratio of the open area of a printing aperture to the area of the walls of the printing aperture, whereby, as the area ratio decreases, there will be an increasing stiction of the print medium to the walls of the printing aperture and a decreasing stiction of the print medium to the substrate on which the print medium is printed. The area ratio currently dictates the stencil design, in that there is a lower limit to the area ratio, currently about 0.6, which can be used to achieve an acceptable transfer efficiency, typically about 75%.
Achieving an acceptable transfer efficiency is particularly problematic where deposits of a range of sizes are required, from standard size structures to ultra-fine pitch structures, in that standard size structures require the use of a relatively thick printing screen, typically 100 μm in thickness, in order to provide the necessary volume of print medium, but the consequence of using a relatively thick printing screen is that the printing apertures for the finer pitch structures necessarily have a low area ratio, which reduces the transfer efficiency for the deposits printed by those printing apertures. The transfer efficiency of the finer pitch apertures can be improved by reducing the thickness of the printing screen, but this has the consequence of reducing the volume of print medium which can be deposited from the standard size apertures.